In the production of pressedboard, fibrous or particulate materials, e.g. of a cellulosic substance can be formed into layers having defined boundaries, i.e. mats, and subjected to heat and pressure to bond the fibers or particles in the presence of natural or added binders into more or less rigid structures generically referred to as pressedboard.
Depending upon the density of the materials, the degree of heat and pressure, the compression to which the mat is subjected and the nature of the fibers or particles and of the binder, the pressedboard can have a wide range of densities, porosities, rigidities and compressive and tensile strength.
Pressedboard fabricated in this manner can be utilized as insulation, as structural materials, as facing materials and in the fabrication of furniture, cabinetry and the like. It can be laminated with finishing foils or films to have smooth or embossed textures, natural or other patterns and various colors.
In general the mat is formed by depositing the particulate or fibrous material on a press underlay which is transported past the mat-forming stage on a conveyor, the underlay facilitating handling of the non-coherent or loosely coherent mass constituting the layer. This underlay can be used, for example, to carry the layer through stations in which laminates are applied or other treatments of the layer may be carried out and for carrying the layer into or depositing the layer within a press.
The pressedboards which are made in this manner include particleboard and fiberboard and frequently the underlay is composed of a web of a heat- and pressure-resistant material, e.g. metal screening or latticework which additionally applies a pattern to the pressedboard during its formation. In the discussion below reference may be made to this web as a screen since it is most usually a metallic lattice composed of wire, although the term is intended here to include any web which can serve as an overlay of support on which the layer can be built and which can be utilized to transport and/or press the layer.
The production of pressedboard in the manner described requires the use of single-platen or multiplaten presses, systems for charging and discharging these presses, conveyor systems for the transporting of the layers, and devices for forming the layer upon a surface with graded or non-graded dispensing of the fibers or particles.
Such systems may be used as are described in the commonly owned copending applications Ser. No. 127,572 of Mar. 6, 1980 (now U.S. Pat. No. 4,289,467), Ser. No. 207,429 of Nov. 17, 1980 (now U.S. Pat. No. 4,336,008), Ser. No. 238,709 of Feb. 27, 1981 (now U.S. Pat. No. 4,352,256), Ser. No. 259,675 of May 1, 1981 (now U.S. Pat. No. 4,412,801), and U.S. Pat. Nos. 3,565,725, 3,409,942, 3,413,145, 3,396,783, 3,332,819, 3,428,505, 3,241,189, 3,050,777, 3,224,758, 3,017,271, 3,050,200 and 3,860,381. Other art dealing with this subject matter can be found in the United States Patent Office Manual of Classification, classes and subclasses to which these patents and applications are assigned and in the files thereof.
Conventional systems in which the layer is built upon a flexible underlay have been found to have problems which have resulted in irregularities in the mat and hence irregularities in the pressedboard which results therefrom. Specifically, difficulties are encountered with the displacement of the underlay, generally where the fiber or particle-dispensing unit is stationary, because of bowing, wrinkling or folding of the underlay.